Prior art clock generation and distribution schemes for emulation systems typically include a base clock signal, circuitry for frequency multiplying or frequency dividing the base clock signal to generate derived clock signals and circuitry for distributing the derived clock signals. The derived clock signals are typically related to the base clock signal by powers of two. For example, the base clock signal may be frequency divided by two and frequency multiplied by two in order to provide three synchronized clock signals having different frequencies. To generate these derived clock signals, dedicated circuitry is provided to generate each derived clock signal. Additional or different clock signals require additional or different circuitry.
Thus, prior art clock generation and distribution schemes are rigid with respect to the number of derived clock signals provided and the relationship of the derived clock signals to the base clock signal. In particular, prior art clock generation and distribution schemes are unworkable for emulation systems that employ many clocks.
When providing multiple clock signals derived from a single base clock signal, the emulation system typically starts, stops and resumes the base clock signal to start, stop and resume emulation. However, derived clock signals may not be in phase with the base clock signal. When the derived clock signals are not in phase with the base clock signal and emulation is stopped, emulation stops with respect to a rising or falling edge of the base clock signal. However, in clock domains operating on derived clock signals emulation continues until a subsequent derived clock edge. When emulation is resumed, the base clock signal resumes where stopped, however, because the derived clock signals may not be stopped at the same point in time as the base clock signals because the derived clock signals may be out of phase with respect to the base clock signal, the derived clock signals may not resume at the point where emulation was stopped. Therefore, these prior art clock distribution schemes may not provide fully functional start, stop and resume functionality for emulation.
Therefore, what is needed is a clock generation and distribution method and apparatus that allows generation of derived clock signals without specific circuitry for each derived clock frequency that allows derived clock signals to resume where stopped whether or not the derived clock signal is in phase with the base clock signal.